Political constraints: Any U.S. President is legally constrained from withdrawing from NATO without Congressional approval, as required by the 2024 National Defense Authorization Act. However, the legislation's legality is not fully settled, and a President could weaken the alliance by ignoring commitments. A potential lack of U.S. intervention in a Russian military expansion, as specified in the premise of the question, would cause outrage primarily from NATO allies and Ukraine, as well as from within the U.S. by security officials, the military, and some political factions. However, this scenario remains hypothetical, as U.S. policy toward NATO is contested and Russia faces challenges in recreating its former sphere of influence. NATO allies and Europe Political leaders: European leaders who have long relied on U.S. commitment to NATO's collective defense principle (Article 5) would likely express strong disapproval. Eastern European states: Countries on NATO's eastern flank, which feel the most vulnerable to Russian aggression, would be especially alarmed. In September 2025, after a Russian drone reportedly entered its airspace, Poland invoked Article 4 for consultations, reflecting heightened anxiety. The alliance's future: U.S. inaction could effectively undermine or dissolve the alliance, pushing Europe to find alternative security models. Ukraine Abandonment: Ukraine, which has sought closer ties with NATO and relied on Western support, would view inaction as a betrayal. Recent developments, such as Donald Trump's September 2025 statement questioning Ukraine's ability to win back territory, have already prompted shifts in U.S. policy that would cause great concern. Future aspirations: Any U.S. deal that pressures Ukraine to cede territory or renounce NATO membership would be viewed as a profound abandonment of its sovereignty. Within the United States Bipartisan establishment: Many members of both the Republican and Democratic parties have historically backed NATO. Figures like Senator Marco Rubio have championed legislation to prevent a unilateral withdrawal from the alliance, demonstrating broad support for its core purpose. MAGA movement factions: Even some within Trump's own base could react negatively. In July 2025, for example, some conservative commentators expressed anger over plans to send weapons to NATO, viewing it as a betrayal of promises to end U.S. involvement in foreign wars. Military and security officials: Senior leaders, such as the Supreme Allied Commander Europe, would likely be concerned about the impact of undermining the alliance and what it means for U.S. force deployment and military readiness in Europe. European preparedness: Growing U.S. pressure has already prompted Europe to consider strengthening its own defense capabilities. In June 2025, NATO members agreed to a new defense investment plan, potentially paving the way for Europe to play a larger role in its own defense. Russia's limitations: Russia is not seen as the same imperial force as the former Soviet Union. Russia has its own internal challenges, and recreating the former empire is not a guaranteed outcome. While the question describes a highly controversial and hypothetical scenario, the potential outrage would come from multiple sources, primarily from NATO allies and Ukraine, as well as bipartisan political and military figures in the United States. Recent U.S. government actions and shifts in policy have already prompted some of these dynamics, highlighting the existing tensions around the future of NATO and Western Europe. MAGA movement factions: Even some within Trump's own base could react negatively. In July 2025, for example, some conservative commentators expressed anger over plans to send weapons to NATO, viewing it as a betrayal of promises to end U.S. involvement in foreign wars. Military and security officials: Senior leaders, such as the Supreme Allied Commander Europe, would likely be concerned about the impact of undermining the alliance and what it means for U.S. force deployment and military readiness in Europe. https://thehill.com/policy/international/5522862-lavrov-nato-eu-russia/?fbclid=IwVERTSAND2EZleHRuA2FlbQIxMAABHoi1R5cFyqT2GzjwL0VHIY3Yy_544_yY9DZld8QgwMnD2KFKuVAnMVRgfpap_aem